Light olefins such as ethylene, propylene and butylene serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds.
An alternative route for manufacturing light olefins is to catalytically convert oxygenates such as methanol to an olefin product. This conversion can be accomplished using a variety of molecular sieve catalysts. Several of these catalysts are highly selective in forming ethylene and propylene. However, a significant amount of water is formed as a natural by-product, and this water must be removed from the desired light olefin product.
U.S. Pat. No. 6,740,791 B2 discloses a process for converting an oxygenate to an olefin product. Water is removed from the olefin product using a quench tower to cool the product and condense the water. The olefin portion of the olefin product is recovered in an overhead stream from the quench tower, and the condensed water fraction is recovered as a bottoms stream. The condensed water fraction is recovered from the quench tower, and sent to a fractionator for separation into further streams.
U.S. Pat. No. 6,459,009 B1 discloses an alternative process for converting an oxygenate to an olefin product. Water is removed from the olefin product using a two-stage quench tower to cool the product and condense the water. The olefin product is passed to a first stage quench tower and contacted with an aqueous stream and a neutralizing agent introduced at the top of the quench tower to provide a hydrocarbon vapor stream and a first stage bottoms stream containing water. The vapor stream is cooled and sent to a second stage tower to further separate light olefins and additional water. A portion of the first stage bottoms stream is withdrawn as a drag stream, and another portion of the first stage bottoms stream is recycled to the first stage tower.
U.S. Patent Publication No. 2006/0047175 discloses condensing and removing water and heavy hydrocarbons from an olefin stream containing light olefin compounds such as ethylene, propylene and butylene, and recovering the light olefin compounds. The olefin stream is cooled such that a substantial portion of the water contained in the olefin stream is condensed to form a liquid water stream. Condensation is carried out in a vessel, and following condensation, an olefin vapor stream, the liquid water stream and a hydrocarbon stream are removed from the vessel. The olefin vapor stream that is removed from the vessel is preferably a light olefin stream, preferably comprising one or more olefins selected from the group consisting of ethylene, propylene and butylene.
Removing water and other condensable materials, including catalyst particles that may be present in the olefin product, from an olefin stream can be problematic. Water and catalyst particles are particularly difficult to remove, since hydrocarbon by-product oil is also present and this oil has a tendency to adsorb onto the catalyst particle surface. Although the catalyst particles have a density higher than water, the adsorption of the oil onto the catalyst surface makes the catalyst particles much more difficult to separate from the water, particularly if the oil does not form a clearly separate layer from the water layer, which is often the case. This adsorption of oil on catalyst particles and poor separation of oil and water layers can cause further problems in downstream processing, including increased fouling of equipment and enhanced problems in wastewater treatment.
More efficient systems and methods for separating the condensable materials from the light olefin compounds are desirable. Such systems should be low in maintenance and easy to operate. There is, therefore, a need to pursue more efficient ways of removing water and other condensable materials from olefin streams, particularly olefin streams high in light olefin content.